1. Field of the Invention
The present invention relates to new cis-lactones, to processes for their preparation and to their use for the separation of cis/trans isomers of pyrethroid carboxylic acids.
2. Description of the Prior Art
Synthetic pyrethroids combine exceptionally good insecticidal properties with a very low mammalian toxicity. They are of considerable interest to the agrochemical industry and much effort has been expended in finding economic routes to them and to their principal intermediates.
The general formula of one class of these pyrethroids may be represented as follows: ##STR3## wherein each asterisk denotes an asymmetric carbon atom, each of the groups X.sup.1 and X.sup.2 represents a fluorine, chlorine or bromine atom or a hydrocarbyl group and R is a radical known to impart insecticidal activity to the molecule, for example 3-phenoxybenzyl or alpha-cyano-3-phenoxybenzyl. It is known that the stereo-isomeric form of the acid part of the ester of formula I should be 1R,cis for maximum insecticidal activity, i.e. the absolute configuration at carbon atom 1 is R and the two hydrogen atoms on carbon atoms 1 and 3 are in a cis relationship. This nomenclature is known as the Elliott nomenclature and is defined in M. Elliott et al., Nature, 1974, 248, 710.
It follows, therefore, that if these cis esters of formula I are to be prepared, it would be useful if one could dispose of a process for the isolation of cis carboxylic acids of the general formula: ##STR4## wherein X.sup.1 and X.sup.2 have the same meaning as in formula I-from mixtures of cis and trans carboxylic acids of formula II would be useful. A novel intermediate has now been found in such an isolation process.